Protecting magnesium and its alloys from fire



Patented Dec. 25, 1945 MAGNESIUM AND rrs ALLOYS mom mm James geld, New York, N. Y., assignor to The Dow Chemical Company, Midland, Michqa corporation of Michigan I PROTECTING No Drawing.

Application February 7, Serial No. 521,435

Claims. (c1. ur-m) The invention relates .to coated articles of magnesium andits alloys. It more particularly concerns a method of protecting magnesium articles from ignition by fire by liquid film-forming compositions adapted to produce coatings on magnesium and its alloys as by painting, varnishing, enamelling, and the like. A

In this description and the appended claims, the term magnesium is used to mean either magnesium or alloys thereof in which magnesium is present in an amount constituting not less than about 75 per cent of the alloy. The term liquid film-forming composition is intended to include paints, varnishes, lacquers, enamels, and like compositions forming coatings.

Magnesium is now widely used in the manutacture of articles where its lightness is of prime importance as in the field oftransportation, and more particularly that of aircraft. In these applications, magnesium is used in a wide variety of forms including castings,'-extruded shapes,

forgings, and rolledshe'et. In these forms, magnesium is more or less susceptible to ignition upon being'heated in air to a temperature approaching the melting point of the metal. 7 In general, the smaller the cross-sectional area of the object of magnesium the more readily it may be dichromate treatments (U. S. Navy Bureau 0! Aeronautics designations M303 and M407, re-

spectively) and preferably before other paints or varnishes are applied so that the fluoride ingredient is brought close to the metal surface as w'henthe fluoride ispresent in the primer.

The fluorides which may be used are those having a higher melting point than magnesium and solubility of preferably less than 0.7 gram per 100 cc. of water, viz.: magnesium fluoride, chromic fluoride, calcium fluoride, lithium fluoride, manganese fluoride, lead fluoride, strontium fluoride calcium fluoride, magnesium fluoride, strontium fluoride,' manganese fluoride, chromic fluoride,

brought to the ignition temperature and thus set of the material not hitherto adequately solved.

I have now found that by adding to a paint,"

, varnish, or like coating or film-forming composition, an inorganic fluoride having a melting point above that ot'magnesium and preferably not readily dissolved by water, coating compositions are obtained which upon application to articles of magnesium, and drying or hardening in situ, make them resistant to ignition byfire. In addition, fires once started do not as easily propagate on magnesium coated with such compositionsand the compositions may be used also for their decorative and protective value. If desired,'the usual decorative and protective finishes may be applied over the fire-retarding coating.- The flreretarding coating composition may be applied to the magnesiumarticles after conventional chemical surface treatments, such as, for example, the .chrome pickle" and lead fluoride. In applications not subject to exposure to excessive moisture, either aluminum fluoride or beryllium fluoride may be used as the fluoride constituent of the coating composition.

In preparing a liquidrcoating compositmn for treating articles of magnesium according to the invention, any of the ,usual organic-liquid coat-' ing materials may be employed which produce an organic binding film-upon drying, such as those containing in solution a binder that is deposited as the solvent dries or evaporates, or those which contain drying oils. These binders or film-forming materials are present in or formed upon drying conventional paints, varnishes, lacquers, enamels, primer. coatings, etc., as, for example, the linseed, tung, and like drying oil-base pigmented paints; the cellulose, artificial and natural gum and resin-containing lacquers; varnishes, such as those comprising a volatile vehicle carryingin solution a solubl film-forming or binder material; and enamels as,'fo r example, those composed of pigmented varnishes.

In accordance with the invention, there is added to the film-forming liquid coating material, such as-one of the aforementioned coating materials, atleast one of'the aforesaid inorganic metal fluorides in a suitably comminuted form and thoroughly incorporated in the liquid-coating material, asby stirring or preferably grind- .are obtained when the fluoride constituent of ing in a ball or pebble mill, in a proportion of at least one per cent and preferably as much as from five to one hundred per cent'or more by weight of the film-forming solidsoi' the coating material, these solids including the binders, such as the gums, resins or the like, and the pig- 'ments, if any. The fluorides so employed may be ground to 200 mesh or finer, preferably to 320 mesh, although other degrees of fineness may be used. When pigments are to be added to the coating liquid, it is preferable to employ the fluoride in as fine a stateas that of the pigment and to grind both the fluoride and pigment together in incorporating them into the liquid vehicle. In th absence of a pigment in the filmiorming liquid vehicle, the fluoride may be proportioned to'the. binder in the ratio of 2 5 to 60. parts of fluoride per 50 parts of binder and incorporated in the vehicle by grinding in a. ball or pebble mill or o ther suitable device. The following examples are illustrative of the invention: I

Example 1.Prime coating 'Per cent by weight 25 zinc yellow 24 2 .Pigment 'asbestine v I fluoride, e. g. Cal h- 4.3- Ibinder: alkyd resin 26.7 vehicle ,solve'nt: xylol 40.6

Example 2.-One coat finish Pigment (fluoride, e. g. CaF:)-.. 20 v binder: phenol-formaldehyde Vehicle resin 40 solvent: toluol 40 The fluoride-containing liquid coating materials prepared 'as described may be applied by dipping, brushing, or spraying the article of magnesium to be coated and drying as is usual with conventional paints, varnishes, lacquers, enamels and the like, as the case may be. Best results the coating material is in the prime c'oat. In eneral, it is preferable to so coat all the surfaces of the article that are normally exposed and subject to the riskof fire, although coating but one side of a sheet magnesium article, for example, gives valuable protection from ignition by flre. V This application is a continuation-in-part of my copending application Serial No. 281,217, flled June 26, 1939.

I claim: v 1. The method of protecting an article of ma nesium from ignition by fire which comprises applying over, the surface of the article a filmforming organic liquid coating composition having incorporated therein a comminuted metal fluoride having a melting point above that of magnesium and adapted to protect it from ignition by flre, so as to form a coating thereon, and

9 allowing the coating to harden.

2. The method of protecting an article of mag nesium from ignition by flre which comprises applying over the surface of the article a fllmforming organic liquid coating composition containing a pigment with which is incorporated *a comminuted metal fluoride having a melting po nt above that of magnesium and a solubility in water of less than 0.7 gram per 100 cc. of water so as to form a coating thereon, and allowing the coating to harden.

3. The'method of protecting an article of mag I nesium from ignition by-flre-which comprises applying over the surface of the article a fllmforming organic liquid coating composition ha'v-' ing incorporated therein calcium fluoride so as to form a coating thereon, and allowing the coat-- ing to harden.

4. The method according to 3 in which the metal fluoride is magnesium fluoride 5. The method according to claim 3 in which the metal fluoride isa mixture of calcium fluoride and magnesium fluoride.

JAMEB-B. RFID. 

